The invention relates to a cryopump having a casing, a gas inlet to which a vacuum chamber can be connected through a valve, a vacuum pump connected to the casing through a valve, a two-stage refrigerator as cold source situated in the casing, and pumping surfaces which are equipped with an electrical heating means at both of the refrigeration stages of the refrigerator. The invention furthermore relates to a method for the operation of a cryopump of the kind specified. Operation of the cryopump is to be understood in this case to mean not only pumping and evacuation, but also regeneration.
Cryopumps, like ion getter pumps, are of a kind which do not deliver directly to the atmosphere the gases removed from a vacuum chamber but first accumulate them on the pumping surfaces. When their pumping capacity is reached, it is necessary to regenerate the pumping surfaces, that is, to remove the gases that are on the pumping surfaces. This can be accomplished, for example, by shutting off the refrigerator after the valve to the vacuum chamber has been closed or after the preferably heated gases flow through the pump. The warm gases are intended to warm the pumping surfaces and carry away the gases that are set free. In another regenerating method (disclosed in the German published patent application No. P 35 12 614.0) the pumping surfaces are heated by an electrical heating means on the pump surfaces. The liberated gases are pumped away by means of a forepump connected to the pump casing.
The regeneration of cryopumps involves a number of difficulties. On the one hand, it is not always easy to know when the mixture capacity of a cryopump is reached. It is especially difficult to know whether any residual capacity still available will suffice for the next pumping cycle. This problem is present, for example, in the use of cryopumps on vapor depositing or sputtering systems. In systems of this kind a batch is placed in a vacuum chamber which is then evacuated by means of the cryopump. Then reactive gases and/or inert gases are additionally admitted, up to a pressure at which the coating of the parts is performed. After the admission of gas has been interrupted, the remaining gases are removed in order to check the previous vapor depositing step. Then the vacuum chamber is separated from the cryopump and aired for the next batch. Whether the cryopump still has sufficient capacity after the last evacuation can be learned only with difficulty. Usually, for reasons of safety, a regeneration is started long before the maximum capacity is reached. For this period of time the operation of the system must be interrupted.
Furthermore, it is difficult to know when a regeneration has ended, i.e., when the pumping surfaces are completely freed of the gases by heating. It is therefore common practice to assume maximum plate loading and to heat for a corresponding length of time. This, however, involves the disadvantage that the cryopump is unavailable for the evacuating operation for a relatively long time, and thus the whole system to which the cryopump is attached often is out of operation for an unnecessarily long time.
The present invention is addressed to the problem of equipping a cryopump of the kind described above with monitoring and controlling systems such that the time expended for regeneration purposes is minimized.